Slow charging !!

Hey everyone, i truly need your suggestion. I've just bought awhite Lumia 900 for more than 3 weeks, It's been okay, not any problems found. However, it took me more than 8 hours to charge fully yesterday and the battery was able to use less than 12 hours ( even i deactivated 3G or wi-fi connection either ).

I noticed that it took 1h per 10% battery while charging. After being fully charged, the estimated remaining time is around 16 hours. It used to be more than a day remaining time. I haven't change any setting ( i don't play games, listening music less than hour, automatic bright screen, not roaming... )

It's really inconvenient to charge a phone 2 times a day. Any one have any experiences or the same situation as mine ? I really do appreciate for your helps, thank you so much ! Could you please email me in case you have a solution ? trungnguyen0313@gmail.com

I'm learning English so there are some grammar problems, hope you don't mind.